I did some empirical experimenting with the airbox tonight. As some of you may know, the airbox consists of a few parts:
- The airbox itself, which encloses the throttle body and is draped from under the front of the seat all the way to the headstock.
- The airfilter element, which slots into the the top/front of the airbox. This is where the airbox gets sealed.
- Next there is the cover for the air filter element, which job seems to be mostly to reduce air intake snarl.
- And finally the faux tankcover that makes the bodywork on the front fit, and which in its turn is locked up by the white side covers.
When I unbolted the side panels and the faux tank cover I found out there is very little room to breath through. The inlet duct, which is formed by the top of the airbox (1), and the inside of the air filter cover (3) is around 1" x 2" , maybe even smaller.
I figured the next.
- The way engine breathes is decided by many parameters of which the shape and volume of the airbox are very important.
- On the G series the airbox is sealed with the air filter element, which in its turn sort of chokes the air that enters the airbox.
- I don't want water and dust to fall directly into the airbox. So I do not want to open up the faux tank cover with metal mesh or something.
The easiest way to see how the engine would respond to more air was to simply leave the air filter cover (3) off and leave the rest as it is. So the faux tank cover will stay in place, and leave the bodywork near the left fork leg as it is. All very stealth, and nothing got destructed. The airbox is still completely sealed by the air filter element.
Anyway, bolted the rest of the pieces back together. Btw, mr BMW designer will you please explain to us why the side panels are fitted with hex-head bolts, and the faux tank cover with torx head bolts. They are even interchangeable!!! Aaaarrgghh!!!!!!!!!!! Why!!????
Ok I fired up the bike and somehow expected the same behaviour as leaving the airbox sidecover off on my DR. Hard idle and no pick up on the revs because of a too lean condition. Not so on the G. It idled completely normal and gave a nice sweet "murbling" sound. Left it standing for a few minutes to get it warmed up a little. Took my jacket/helmet and went for a ride.
First thing I noticed was a nice throaty snarl. Next thing was the engine pick up from low revs. Nearly too much in the rain, in second gear the rear wheel would simply spin if I gave it some throttle. Very rewarding! I could not find any downsides. no jerking, no bogging, no stalling no whatsoever. Just a very eager engine and a naughty snarl!
As I was afraid my enthousiasm was mostly fed by the inlet snarl, I changed back and forth between with and without the air filter cover. The wet roads were the deciding judge. With the air filter cover fitted I had a very good accelleration, just as I was used to of course. Without the airbox cover though, the rear wheel would simply spin if I janked the throttle. First I though I was fooling myself, but doing repeated roll-on tests proved to me the midrange and especially the throttle response had improved. I cannot tell if the pure horsepower have improved, but the eagerness, the willingnes to pick up at opening the throttle (and the willingnes to lift the front wheel!) has improved definately.
"Aren't you afraid your McGuyvering is leaning off the carburation?
" Good question, I was asking the same myself. But I am not afraid I am. There are many sensors around the engine to deliver the best air-fuel mixture. The barometric pressure will be lower at 10.000 feet height for example, and a clogged air filter element has to be compensated for as well. The engine also uses a lambada sensor which is used to keep the mixture at the stoichiometric optimum. All in all, as long as I am on a steady throttle I'm not afraid the engine will run too lean. On accellerations the mixture simply isn't too lean, for then it wouldn't accellerate as it does now. Instead it would bog.
All in all, I'm going to keep it like this for a while. When my rear tire is worn I will maybe do two dyno runs. One with, and one without the cover (choker??). Just out of curiousity.
Btw, this bike needs no aftermarket exhaust. As it is, especially with the inlet snarl, it sounds very naughty. (which reminds me, I could ask a friend to cut open the exhaust, remove the catalytic converter and weld it back up again, totally stealth).
If we can work something out with the foam airfilter i would greatly appreciate it. In the meantime I've asked my local dealer and Touratech importer to see what he can do.
Keep up the good work all!!